The comma is one of those punctuation marks that has an outsized impact in relationship to its size. It’s the little but mighty mark of the punctuation world.
One place where a comma can make a big difference is in restrictive and nonrestrictive—or if you prefer—essential and nonessential clauses. Essential/restrictive clauses include necessary information that must not be set off with punctuation, such as these examples:
Jan and Perry bought the clock that chimes on the hour and half hour.
The fabric (that) Jenny needed to finish her project was backordered.
A theater that has served a small southern Missouri town for fifty years will close next month.
Nonrestrictive or nonessential clauses or phrases are set off with commas. Note how a slight change in the wording and punctuation changes the meaning of these sentences:
The clock, which chimes on the hour and half hour, was Jan and Perry’s anniversary gift.
Here the description (modifier) of the clock is not essential to the sentence and is set off in commas.)
The fabric, which was backordered, would not arrive before the project deadline.
The Omni Theater, which has served a small southern Missouri town for fifty years, will close next month.
Making the clause nonrestrictive indicates that the Omni is one of several theaters in the small town. The previous sentence with its restrictive clause limits the closing to the theater that has served the town for fifty years.
Note the use of that and which. That is used for restrictive clauses, which for nonrestrictive. Nonrestrictive clauses could be eliminated without changing the basic meaning of a sentence, thus they are set off with commas.
Similar rules apply to appositives.
An appositive is a word or phrase modifying or describing a noun. A nonrestrictive appositive restates the noun or pronoun without changing the essential meaning of the sentence.
Peggy’s husband, Robert, is a model train aficionado.
Since Peggy has only one husband, the appositive Robert is nonessential and is set off in commas.
My brother John is five years younger than me.
I have more than one brother so it is essential to identify which one is younger. Here, John is a restrictive appositive; no comma.
Fredrick Backman’s debut novel, “A Man Called Ove,” is one of my favorite books.
An author can only have one debut novel, so either debut or the book title could be eliminated without changing the basic meaning of the sentence.
Backman’s novel A Man Called Ove has been on the New York Times Best Sellers list for more than forty weeks.
Backman’s debut novel is the only one to have been on the best seller list this long, thus the title restricts the modifier to that book and no comma is required.
Like language itself, punctuation rules are evolving—especially in the age of social media. But the publishing world still adheres to these. Aspiring authors ought to, also.
